I used tap water in my old tank, no real problems, biggest issue was that i was putting 5ppm Nitrate into the tank and taking out the same, could never get it below that. Didn't have biopellets running though, now after using them i have no idea why i didn't. There's lots of other funky stuff in tap water though, here's richmond's report:
http://www.richmond.ca/__shared/asse...eport27016.pdf